Computerized tomography (CT) is an advanced method of nondestructive evaluation (NDE) employed in medical imaging, material science, quality assurance etc. An image is recovered from projection data obtained with the CT scanner through a reconstruction algorithm. The CT scanner consists of a source (usually emitting X-rays), a detector, and a turntable positioned between the source and the detector. The object to be investigated is placed onto the turntable and is rotated 360.degree. while the projection data (i.e., the X-ray shadow) is recorded. FIGS. 1a and 1b illustrate the scanner setup for 2D and 3D CT. While in medical imaging the scanner revolves around the patient, in industrial imaging the investigated object is rotated on the turntable. The projection data is recorded with the resolution made available by the employed detector.
The resolution of the detector is limited due to technical reasons, where the cost of the detector grows with increasing resolution. However, investigation of tiny objects requires projections with a high resolution, since projections of low resolution result in blurred images. Besides using high resolution detectors, existing methods of CT microscopy further increase the effective beam spread by moving the object/turntable closer to the point source as depicted in FIG. 2 where the detector is perpendicular to the center X-ray beam. The reconstruction algorithms become inherently instable for reconstruction close to the source, and tend to greatly magnify noise and other artifacts (see FIG. 5a and 5b).
The image of the object is recovered from its projections with a reconstruction algorithm. The standard fan (2D) and cone (3D) beam convolution backprojection algorithms most commonly used in practice require the central ray emitted by the source which intersects with the axis of object rotation to be perpendicularly incident on the detector center. Any deviation from this required geometry results in images that are at least degraded and sometimes completely useless.
Some methods of high resolution CT have been developed to date. These methods are either based on the use of expensive high resolution detectors or close proximity CT (moving the object close to the source) using standard reconstruction algorithms, or sole software solutions preprocessing the projection data to better utilize the inherent resolution information. A hybrid method combining a modified projection geometry (hardware) with a reconstruction algorithm adapted to the new geometry (software) has the potential to outperform any of the previously found methods.
The method of the present invention has many uses. For example, the method can be used in related techniques such as ultrasonic inspection, diffractive tomography, impedance tomography, MRI scans, medical imaging, etc. One of the most desirable uses for computerized tomography is in the field of material science. For example, it is very difficult to perform non-destructive microstructural analysis and quality review on composite materials. These materials are being used in an increasing number of applications and have already become the preferred material for high-performance structural components in the aerospace industry. A similar development can now be observed in the automotive industry, where composite materials are expected to replace any traditional materials due to superior strength-to-weight ratio, stress behavior, life-expectation, manufacturability, etc. For the further development of composites and their successful integration in the manufacturing process, the availability of high-quality methods for testing, evaluation, and verification of these materials, especially their microstructure, is imperative. CT is an investigative tool providing an advanced method of material testing and evaluation. However, the presently available techniques for performing CT are time consuming and, to achieve the necessary high resolution, expensive detectors are required. Therefore, to provide an economical procedure for high resolution microtomographic investigation of materials, a new method of performing CT had to be developed. Moreover, due to the high cost of high resolution detectors, it was clear that standard detectors would have to be employed.